This invention relates to an in-situ permeability test using bore holes that is performed for the purpose of investigating the dynamic and hydraulic properties of crevices that serve as passages for underground water and, in particular, to a low-water-pressure controlled hydrologic test method in which the pressure in a measurement pipe is measured after establishing a certain water level in the pipe.
In a conventional JFT (Johnson Formation Test) method for measuring the permeability coefficient of ordinary rocks, a measurement pipe for water-level observation is inserted into a bore hole which has been bored into an aquifer. It is noted that the Johnson Formation Test is a nonsteady permeability test using double packer. Packers are provided in the lower section of the measurement pipe, and the permeability coefficient of the rock concerned is obtained from the rate at which the water level within the measurement pipe rises for the purpose of investigating and analyzing the crevices that serve as the passages for underground water.
FIG. 7 illustrates a conventional JFT test method. The reference numerals in the drawing respectively indicate the following: 31: bore hole; 32: measurement pipe; 33: strainer 34, 35: packers; 36: trip valve; 37: water level measuring element; 38: tester; 39: piping; 40: pressure control box; 41: go-devil; and 42: underground water level.
The measurement pipe 32 shown is closed at its front end, the packers 34 and 35 being provided around the lower section of the measurement pipe 32 with the strainer 33 between them. The trip valve 36 is provided in the upper section of the measurement pipe 32, and serves to prevent underground water from entering the pipe. The water level measuring element 37 inserted into the measurement pipe 32 is connected to the tester 38. The piping 39 for sending air under pressure connects the packers 34 and 35 to the pressure control box provided outside the measurement pipe 32.
As shown in the drawing, the strainer 33 is lowered together with the packers 34 and 35 to a position within the bore hole 31 where the permeability coefficient is to be obtained, air being conveyed under pressure by operating the pressure control box 40 so as to expand the packers 34 and 35, which seals in any spring water in the bore hole 31. Next, the tip of the go-devil 41 is hit against the trip valve 36 to open it instantaneously, which causes the underground water below the packer 34 to flow through the strainer section into the measurement pipe 32 and rise therein. This rising water level is electrically measured with the passage of time by means of the water level measuring element 37, the permeability coefficient being obtained from the elevated water level and the time that passes using Hvorslev's analysis equation, as follows, for the single-hole-type permeability test: EQU K=(2Rw).sup.2 In(mL/ra)/{8L(t2-t1)}In(H1/H2) (1)
where
K: horizontal permeability coefficient (cm/s); PA1 Rw: inner diameter of the measurement pipe (cm); PA1 ra: diameter of the boring hole (cm); PA1 L: length of the measurement section (cm); PA1 m: permeability coefficient ratio in the vertical and horizontal directions (usually 1); and PA1 H1, H2: water levels t1, t2 (sec) after the water level rise start (cm).
The value In(H1/H2)/(t1-t1) in the above equation is obtained from the inclination of the linear section of a relationship curve of t-InH which is drawn on a semilogarithmic coordinate sheet whose ordinary scale represents the time t and whose logarithmic scale the water level H.
By conducting measurement by this conventional JFT method until the underground water level attains equilibrium, the pore water pressure in the aquifer can be obtained from the water level subsisting at that time.
In a permeability test conducted by this conventional JFT test method, however, it is necessary to recover the trip valve each time the measurement depth is changed. That is, the measurement pipe has to be drawn up for each measurement, resulting in very low efficiency, particularly in a case where measurement for various depths is conducted within a deep bore hole. Moreover, the water hammer effect involved inevitably subjects the rock to dynamic damage, so that the condition of the rock will change. In addition, due to the great difference in head pressure, the clay in the rock crevices is displaced to cause clogging, resulting in a substantial lowering of the measurement accuracy. Furthermore, the measurement is conducted under a high water pressure that would not be generated under natural conditions. That is, the conditions under which the measurement is conducted are different from the natural state. Besides this, the t-logH curve obtainable with the present level of measurement techniques is mostly a curved line, so that the analysis will not reflect the actual state. In the case of an aquiclude, recovery of water level takes a long time, so that the measurement of the pore water pressure that is necessary for the analysis is inevitably a very time-consuming operation.